The invention relates to a method and apparatus for the coded transmission of messages by splitting up the clear (i.e. uncoded) signals to be transmitted into elements of equal length, which are transposed at the transmitting end by being delayed by at least partially different times and are transposed back at the receiving end by being further delayed by at least partially different times. The consecutively numbered elements e.sub.1, e.sub.2, e.sub.3, . . . of the clear signal x have coinciding lengths T.sub.o (see FIG. 1), and their transposition in time leads, for example, to the coded signal y, of which the first element e.sub.4 appears undelayed at the moment t = 3T.sub.o, while the other elements appear with varying delay. After transmission of the signal y at a receiver, the elements are restored to their original position by retransposition in order to recover the original clear signal.
The elements e.sub.1, e.sub.2, . . . may, as shown in FIG. 2, be pulses of the duration T.sub.o, which are keyed between -1 and +1 or between 0 and 1 in accordance with a telegraphic message. Each element may, however, also comprise a plurality of individual pulses of a data signal s, as shown in FIG. 3. The pulses may also be quantized in a plurality of stages. The formation of elements, the amplitude of which corresponds to the scanned values, formed at intervals T.sub.o, of a continuously variable clear signal s (t), is shown in FIG. 4. Instead, however, sections of the clear signal s (t) of constant length T.sub.o may be formed as elements e.sub.1, e.sub.2, . . . as shown in FIG. 5. FIG. 5 also indicates that, instead of these continuously variable signal sections, a train of short individual pulses c (t) is suitable for forming the elements (see element e.sub. 3). Now, as a result of the encodingprocess, the sequence of such elements in time is altered, while the nature of the individual elements can remain unaltered.
Methods and devices for time coding, that is to say for the transposition in time of message elements, have become known for example through the Swiss Pat. No. 212,742 and 232,786, which describe how omissions and also repetitions of individual elements are avoided by periodically actuated switches. A periodic repetition of the transposition program effected at short intervals is undesirable, however, for cryptologic reasons. Accordingly, in the Swiss Pat. No. 518,658, a method is described which renders possible the control of the transposition process by random signals, as a result of which, periodic repetitions of the transposition program during a transmission are avoided. This control is achieved by means of a separate position register which, however, considerably increases the total expenditure necessary. The total expenditure on known devices is also comparatively heavy because the storage devices used are generally only partially filled with message elements wherein at least 50% of the stored locations remain unoccupied at any moment.